Switch pure HTTP scraping to some tool which supports javascript evaluation, such as Capybara (with proper driver selected). This can be slow, since you're running headless browser under the hood plus you'll have to set some timeouts or figure another way to make sure the blocks of text you're interested in are loaded before you start any scraping.

Second option is to use Web Developer console and figure out how those blocks of text are loaded (which AJAX calls, their parameters and etc.) and implement them in your scraper. This is more advanced approach, but more performant, since you won't make any extra work, like you've done in option 1.

Have a nice day!

UPDATE:

Your code above doesn't work, because the response is HTML code wrapped in JSON object, while you're trying to parse it as a raw HTML. It looks like this:

require 'json'
json = JSON.parse(open(url).read) # make sure you check http errors here
html = json['markup'] # can this field be empty? check for the json['error'] field
doc = Nokogiri::HTML(html) # parse as you like

I'd also advise you against using open-uri since your code may become vulnerable if you use dynamic urls because of the way open-uri works (read the linked article for the details) and use good and more feature-wise libraries such as HTTParty and RestClient.